Preserve it from 8-3. Kill the Hammer Bros. with fireballs from a distance. 8-3 has a few power-ups you can collect. For the one near the end of 8-4, if you don't have a Fire Flower, wait for the Hammer Bro to jump, then quickly run under him and jump the gap.
I wonwith uigi better he jups higher this efter goes al the way to d-4 to get tos eond princess to rescue Mario thunder and lightning and rain there to destract you
